Solid-state image sensors such as CCD image sensors and MOS image sensors are widely used as devices for obtaining video information on subjects in video cameras, digital cameras, and the like. A solid-state image sensor includes a pixel section in which a plurality of pixels with photoelectric conversion regions are arranged in a two-dimensional array. The photoelectric conversion region in each of the pixels performs photoelectric conversion on light incident on the pixel from a subject, and accumulates signal charge generated by the photoelectric conversion. This signal charge is output from a signal readout circuit, thereby obtaining video information.
The pixel size of solid-state image sensors has been reduced in recent years, resulting in higher resolution. The reduction in pixel size, however, reduces the ratio of the light-receiving area per a unit pixel area (i.e., fill factor), and also reduces the area of photoelectric conversion regions. Consequently, the sensitivity of the solid-state image sensors decreases. Since the light-focusing ability of a microlens to long-wavelength light is lower than that to short-wavelength light, the reduction of the fill factor reduces the sensitivity especially to long-wavelength light. In addition, as the light wavelength increases, the light absorption efficiency in a semiconductor substrate decreases, and thus the sensitivity of the solid-state image sensor to long-wavelength light decreases. Enhancement of the sensitivity to infrared light having a long wavelength is essential for solid-state image sensors capable of capturing infrared light for use in vehicle onboard systems, security systems, and the like.
In view of this, Patent Document 1 describes a solid-state image sensor in which a plurality of photoelectric conversion regions are provided at different depths in a substrate within a single pixel in order to enhance the efficiency in using incident light and, thereby, increase the sensitivity of the solid-state image sensor.
Patent Document 2 describes a solid-state image sensor in which a lower region of a photoelectric conversion region is extended in an in-plane direction (i.e., in the direction parallel to a semiconductor substrate surface) of a semiconductor substrate in order to increase the sensitivity of the solid-state image sensor. Specifically, the extension of the photoelectric conversion region means an increase in the area of a charge collection region, and allows photoelectric conversion to be performed on an oblique component of incident long-wavelength light which was not conventionally absorbed in a photoelectric conversion region and was transmitted to a deep portion of the semiconductor substrate, resulting in an increase in the sensitivity.
Patent Document 3 describes an example of a solid-state image sensor capable of capturing visible light and infrared light at the same time. In this document, four types of color filters are regularly arranged on pixels, and difference operation is performed on outputs from the respective pixels so as to allow a single solid-state image sensor to capture both visible light and infrared light.
Citation List
Patent Document
PATENT DOCUMENT 1: Japanese Patent Publication No. 2007-66962
PATENT DOCUMENT 2: Japanese Patent Publication No. 2001-185711
PATENT DOCUMENT 3: Japanese Patent Publication No. 2002-142228